This is actually exactly the case I was thinking of. We have a few applications that use vault in a much more in-depth way than fetching a couple of secrets, so have the need to interact with it directly. We then have the much more common case of applications that use vault to fetch their database credentials, and for those we use a sidecar to fetch them at startup and another to renew them every 30 minutes.
The 2x sidecars do with 150 lines of YAML configuration what could be done with a library and 10 lines of java. And I don't buy the other theoretical benefits either. Easier to update? Each service can reference the library centrally from our monorepo whereas the YAML is copy-pasted to every service. It's also statically type-checked. Polyglot? Yeah, fair, but we're an almost entirely JVM shop.
Some of this could maybe be made easier with something like Kubevela but I don't think you're actually eliminating any complexity that way, just hiding it.
